diff mbox

[3/6] bdi: Use refcount_t for reference counting instead atomic_t

Message ID 20180703200141.28415-4-bigeasy@linutronix.de
State New, archived
Headers show

Commit Message

Sebastian Andrzej Siewior July 3, 2018, 8:01 p.m. UTC
refcount_t type and corresponding API should be used instead of atomic_t when
the variable is used as a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free situations.

Cc: linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/backing-dev-defs.h |  3 ++-
 include/linux/backing-dev.h      |  4 ++--
 mm/backing-dev.c                 | 12 ++++++------
 3 files changed, 10 insertions(+), 9 deletions(-)

Comments

Sebastian Andrzej Siewior July 16, 2018, 12:30 p.m. UTC | #1
On 2018-07-03 22:01:38 [+0200], To linux-kernel@vger.kernel.org wrote:
> refcount_t type and corresponding API should be used instead of atomic_t when
> the variable is used as a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free situations.

Andrew, is it okay for you to collect this one (and 4/6 of this series,
both bdi)? The prerequisites are already in Linus' tree.

> Cc: linux-mm@kvack.org
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  include/linux/backing-dev-defs.h |  3 ++-
>  include/linux/backing-dev.h      |  4 ++--
>  mm/backing-dev.c                 | 12 ++++++------
>  3 files changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index 24251762c20c..9a6bc0951cfa 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -12,6 +12,7 @@
>  #include <linux/timer.h>
>  #include <linux/workqueue.h>
>  #include <linux/kref.h>
> +#include <linux/refcount.h>
>  
>  struct page;
>  struct device;
> @@ -75,7 +76,7 @@ enum wb_reason {
>   */
>  struct bdi_writeback_congested {
>  	unsigned long state;		/* WB_[a]sync_congested flags */
> -	atomic_t refcnt;		/* nr of attached wb's and blkg */
> +	refcount_t refcnt;		/* nr of attached wb's and blkg */
>  
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	struct backing_dev_info *__bdi;	/* the associated bdi, set to NULL
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index 72ca0f3d39f3..c28a47cbe355 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -404,13 +404,13 @@ static inline bool inode_cgwb_enabled(struct inode *inode)
>  static inline struct bdi_writeback_congested *
>  wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  {
> -	atomic_inc(&bdi->wb_congested->refcnt);
> +	refcount_inc(&bdi->wb_congested->refcnt);
>  	return bdi->wb_congested;
>  }
>  
>  static inline void wb_congested_put(struct bdi_writeback_congested *congested)
>  {
> -	if (atomic_dec_and_test(&congested->refcnt))
> +	if (refcount_dec_and_test(&congested->refcnt))
>  		kfree(congested);
>  }
>  
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 2e5d3df0853d..55a233d75f39 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -438,10 +438,10 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  	if (new_congested) {
>  		/* !found and storage for new one already allocated, insert */
>  		congested = new_congested;
> -		new_congested = NULL;
>  		rb_link_node(&congested->rb_node, parent, node);
>  		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
> -		goto found;
> +		spin_unlock_irqrestore(&cgwb_lock, flags);
> +		return congested;
>  	}
>  
>  	spin_unlock_irqrestore(&cgwb_lock, flags);
> @@ -451,13 +451,13 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  	if (!new_congested)
>  		return NULL;
>  
> -	atomic_set(&new_congested->refcnt, 0);
> +	refcount_set(&new_congested->refcnt, 1);
>  	new_congested->__bdi = bdi;
>  	new_congested->blkcg_id = blkcg_id;
>  	goto retry;
>  
>  found:
> -	atomic_inc(&congested->refcnt);
> +	refcount_inc(&congested->refcnt);
>  	spin_unlock_irqrestore(&cgwb_lock, flags);
>  	kfree(new_congested);
>  	return congested;
> @@ -474,7 +474,7 @@ void wb_congested_put(struct bdi_writeback_congested *congested)
>  	unsigned long flags;
>  
>  	local_irq_save(flags);
> -	if (!atomic_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
> +	if (!refcount_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
>  		local_irq_restore(flags);
>  		return;
>  	}
> @@ -804,7 +804,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
>  	if (!bdi->wb_congested)
>  		return -ENOMEM;
>  
> -	atomic_set(&bdi->wb_congested->refcnt, 1);
> +	refcount_set(&bdi->wb_congested->refcnt, 1);
>  
>  	err = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
>  	if (err) {
> -- 
> 2.18.0

Sebastian
Andrew Morton July 16, 2018, 10:57 p.m. UTC | #2
On Tue,  3 Jul 2018 22:01:38 +0200 Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> refcount_t type and corresponding API should be used instead of atomic_t when
> the variable is used as a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free situations.
> 
> ...
>
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -438,10 +438,10 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  	if (new_congested) {
>  		/* !found and storage for new one already allocated, insert */
>  		congested = new_congested;
> -		new_congested = NULL;
>  		rb_link_node(&congested->rb_node, parent, node);
>  		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
> -		goto found;
> +		spin_unlock_irqrestore(&cgwb_lock, flags);
> +		return congested;
>  	}
>  
>  	spin_unlock_irqrestore(&cgwb_lock, flags);
> @@ -451,13 +451,13 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  	if (!new_congested)
>  		return NULL;
>  
> -	atomic_set(&new_congested->refcnt, 0);
> +	refcount_set(&new_congested->refcnt, 1);
>  	new_congested->__bdi = bdi;
>  	new_congested->blkcg_id = blkcg_id;
>  	goto retry;
>  
>  found:
> -	atomic_inc(&congested->refcnt);
> +	refcount_inc(&congested->refcnt);
>  	spin_unlock_irqrestore(&cgwb_lock, flags);
>  	kfree(new_congested);
>  	return congested;
>
> ...
>

I'm not sure that the restructuring of wb_congested_get_create() was
desirable and it does make the patch harder to review.  But it looks
OK to me.
Sebastian Andrzej Siewior July 17, 2018, 3:56 p.m. UTC | #3
On 2018-07-16 15:57:16 [-0700], Andrew Morton wrote:
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -438,10 +438,10 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
> >  	if (new_congested) {
> >  		/* !found and storage for new one already allocated, insert */
> >  		congested = new_congested;
> > -		new_congested = NULL;
> >  		rb_link_node(&congested->rb_node, parent, node);
> >  		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
> > -		goto found;
> > +		spin_unlock_irqrestore(&cgwb_lock, flags);
> > +		return congested;
> >  	}
> >  
> >  	spin_unlock_irqrestore(&cgwb_lock, flags);
> > @@ -451,13 +451,13 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
> >  	if (!new_congested)
> >  		return NULL;
> >  
> > -	atomic_set(&new_congested->refcnt, 0);
> > +	refcount_set(&new_congested->refcnt, 1);
> >  	new_congested->__bdi = bdi;
> >  	new_congested->blkcg_id = blkcg_id;
> >  	goto retry;
> >  
> >  found:
> > -	atomic_inc(&congested->refcnt);
> > +	refcount_inc(&congested->refcnt);
> >  	spin_unlock_irqrestore(&cgwb_lock, flags);
> >  	kfree(new_congested);
> >  	return congested;
> >
> > ...
> >
> 
> I'm not sure that the restructuring of wb_congested_get_create() was
> desirable and it does make the patch harder to review.  But it looks
> OK to me.

By `restructuring' you mean the addition of return statement instead
using the goto label in the first hunk? If so, then you would have

	refcount_set(&new_congested->refcnt, 0);
	refcount_inc(&congested->refcnt);

which is a 0 -> 1 transition and is forbidden by refcount_t. So I had to
avoid this one.

Thank you applying the patches!

You applied the bdi and userns switch from atomic_t to refcount_t.
There were also the patches
  [PATCH 4/6] bdi: Use irqsave variant of refcount_dec_and_lock()
  [PATCH 6/6] userns: Use irqsave variant of refcount_dec_and_lock()

in the series which make use the irqsave version of
refcount_dec_and_lock(). Did you miss them by chance or skipped them on
purpose?

Sebastian
diff mbox

Patch

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 24251762c20c..9a6bc0951cfa 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -12,6 +12,7 @@ 
 #include <linux/timer.h>
 #include <linux/workqueue.h>
 #include <linux/kref.h>
+#include <linux/refcount.h>
 
 struct page;
 struct device;
@@ -75,7 +76,7 @@  enum wb_reason {
  */
 struct bdi_writeback_congested {
 	unsigned long state;		/* WB_[a]sync_congested flags */
-	atomic_t refcnt;		/* nr of attached wb's and blkg */
+	refcount_t refcnt;		/* nr of attached wb's and blkg */
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 	struct backing_dev_info *__bdi;	/* the associated bdi, set to NULL
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 72ca0f3d39f3..c28a47cbe355 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -404,13 +404,13 @@  static inline bool inode_cgwb_enabled(struct inode *inode)
 static inline struct bdi_writeback_congested *
 wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
 {
-	atomic_inc(&bdi->wb_congested->refcnt);
+	refcount_inc(&bdi->wb_congested->refcnt);
 	return bdi->wb_congested;
 }
 
 static inline void wb_congested_put(struct bdi_writeback_congested *congested)
 {
-	if (atomic_dec_and_test(&congested->refcnt))
+	if (refcount_dec_and_test(&congested->refcnt))
 		kfree(congested);
 }
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 2e5d3df0853d..55a233d75f39 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -438,10 +438,10 @@  wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
 	if (new_congested) {
 		/* !found and storage for new one already allocated, insert */
 		congested = new_congested;
-		new_congested = NULL;
 		rb_link_node(&congested->rb_node, parent, node);
 		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
-		goto found;
+		spin_unlock_irqrestore(&cgwb_lock, flags);
+		return congested;
 	}
 
 	spin_unlock_irqrestore(&cgwb_lock, flags);
@@ -451,13 +451,13 @@  wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
 	if (!new_congested)
 		return NULL;
 
-	atomic_set(&new_congested->refcnt, 0);
+	refcount_set(&new_congested->refcnt, 1);
 	new_congested->__bdi = bdi;
 	new_congested->blkcg_id = blkcg_id;
 	goto retry;
 
 found:
-	atomic_inc(&congested->refcnt);
+	refcount_inc(&congested->refcnt);
 	spin_unlock_irqrestore(&cgwb_lock, flags);
 	kfree(new_congested);
 	return congested;
@@ -474,7 +474,7 @@  void wb_congested_put(struct bdi_writeback_congested *congested)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	if (!atomic_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
+	if (!refcount_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
 		local_irq_restore(flags);
 		return;
 	}
@@ -804,7 +804,7 @@  static int cgwb_bdi_init(struct backing_dev_info *bdi)
 	if (!bdi->wb_congested)
 		return -ENOMEM;
 
-	atomic_set(&bdi->wb_congested->refcnt, 1);
+	refcount_set(&bdi->wb_congested->refcnt, 1);
 
 	err = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
 	if (err) {